The present invention relates to a valve trim cage assembly for control valves in high pressure fluid transfer systems.
In high pressure fluid transfer systems, control valves produce a throttling effect on the fluid flow with consequent reduction in pressure occurring across the valve. Any significant, abrupt pressure drop in such a system may generate severe noise in a gas or cause cavitation in a liquid, i.e., vaporization followed by implosive condensation, with resultant damage to system components due to vibration and/or erosion. The pressure drop across the valve is accompanied by a corresponding increase in velocity of fluid flow through the valve. To overcome the adverse effects of the orifice action of the valve, it has become common practice to so design the valve trim as to break up the flow through the valve into a multitude of small streams which are then led through convoluted paths to produce energy losses in the fluid. This has resulted in a variety of constructions for valve trims, such as stacks of disks, perforated strips wound upon themselves, and cylindrical sleeves nested within one another.
In U.S. Pat. Nos. 3,513,864 and 3,514,074 the flow is subdividied through interconnected openings in disk-shaped grids and through tortuous passages formed in the surfaces of superposed disks. In either case the subdivided streams are led through small, elongated passages with many abrupt turns creating a drag and pressure drop on the fluid.
In U.S. Pat. No. 3,954,124 a valve trim cage assembly is formed from a number of cylindrical sleeves of graduated sizes are nested concentrically. The sleeves are provided with annular passageways or chambers on one surface which serve as expansion chambers and communicate through small restrictive radial orifices to the opposite surface. The restrictive radial orifices and circumferential expansion passageways provide a tortuous path through the sleeve assembly.
The valve trim cages heretofore known have been generally effective in controlling the abrupt pressure drop encountered in high pressure fluid transfer systems, but in many cases have proven to be impractical for commercial use. The complex shapes required have necessitated a variety of different manufacturing processes and painstaking machining to produce the tortuous paths specified. In addition, the concentric sleeved trim cages proposed in the prior art, as illustrated in U.S. Pat. No. 3,954,124, have tiers of wide, deep annular chambers covering one entire surface of each sleeve. These chambers serve as expansion chambers wherein the flow streams exhausting into them from the restrictive radial orifices impinge upon the sidewalls of the chambers causing damage due to erosion. Moreover, frictional losses are minimized by the shortness of the pathway through the radial orifices as compared to the expansion pathway through the annular chambers.